Polymer-stabilized metal colloid solutions, method for producing said solutions and use of the same as catalysts for fuel cell

ABSTRACT

Polymer-stabilized metal colloid solutions, process for preparing them and their use as catalysts for fuel cells. Process for preparing metal colloid solutions by reacting a platinum compound and, if desired, one or more compounds of Rh, Ru, Ir or Pd with a reducing agent. At least one cation-exchange polymer is used for stabilizing the metal colloid solution.

This application is a filing under 35 USC §371 of PCT/EP98/06413, filedOct. 9, 1998.

Polymer-stabilized metal colloid solutions, process for preparing themand their use as catalysts for fuel cells.

The invention relates to metal colloid solutions which comprise one ormore platinum compounds and, if desired, one or more compounds of Rh,Ru, Ir or Pd and are stabilized by polymeric protective colloids, andalso a process for preparing them and their use as catalysts, inparticular in fuel cells.

The use of a sol process for producing heterogeneous catalysts whoseactive centers consist of a metal, in particular a noble metal, forchemical and electrochemical processes is known. Here, a sol of therespective catalytically active metal or, if desired, two or more metalsis first prepared in a separate process step and the dissolved orsolubilized nanosize particles are subsequently immobilized on thesupport. General descriptions of this method may be found, for example,in (a) B. C. Gates, L. Guczi, H. Knözinger, Metal Clusters in Catalysis,Elsevier, Amsterdam, 1986; (b) J. S. Bradley in Clusters and Colloids,VCH, Weinheim 1994, pp. 459-544 or (c) B.C. Gates, Chem. Rev. 1995, 95,511-522.

In general, the sols are prepared using a stabilizer, in particular whenfurther-processable sols having a metal concentration of 0.1% or aboveare required. The stabilizer envelops the metal particle and preventsthe agglomeration of the particles by means of electrostatic or stericrepulsion. In addition, the stabilizer has some influence on thesolubility of the particles.

Possible stabilizers include both low molecular weight compounds andpolymeric compounds.

EP-A-0 672 765 describes the electrochemical preparation of platinumhydrosols using cationic and betaine-type stabilizers and also describescatalysts produced therefrom, which are said to be suitable, inter alia,for fuel cells.

Coated catalysts comprising platinum, which are prepared via acationically stabilized hydrosol and are suitable for fuel cells, aredescribed in, for example, DE-A44 43 701. In these, the particles of anoble metal form a shell which extends up to 200 nanometers into thesupport particle.

Platinum sols containing polymeric stabilizers such as polyacrylic acid,polyvinyl alcohol or poly(N-vinylpyrrolidone) and their use forproducing catalysts, including ones for fuel cells, have likewise beendescribed (J. Kiwi and M. Gratzel, J. Am. Chem. Soc. 101 (1979) 7214; N.Toshima et al., Chemistry Letters 1981 793). Apart from stabilizing thesol in question, the polymers mentioned have no functional importance.

To obtain a low internal resistance in a membrane fuel cell, it is ofcritical importance that the transport of protons and electrons withinthe cell can proceed with as little hindrance as possible. Every barrierbetween the catalytically active platinum centers and the conductionpaths of the electrons and/or protons inhibits the process or brings itto a stop. Critical zones are the sections between the catalyticallyactive centers and the current collectors or the membrane, since aplurality of phase transitions take place in these regions.

A difficult task is to bring about proton charge transport by contactbetween the catalytically active platinum centers and the membrane. Inthe methods known from the prior art, the platinum/carbon mixture is,for example, worked into the surface of the membrane by rolling orpressing. However, this way of establishing the contact is difficult tocontrol and there is a risk of excessive hindrance to mass transfer fromand to the platinum centers. Furthermore, there is a risk of some of theplatinum particles losing contact to the current collector.

In another process, a certain amount of polymeric cation-exchangematerial is introduced into the platinum/carbon layer, for example byimpregnating the platinum/carbon mixture with a solution of thepolymeric cation-exchange material, before the platinum/carbon mixtureis pressed onto the membrane. This process has the disadvantage that itcan result in a reduction in the surface area of the catalyst or thecarbon particles can be enveloped too much so that they becomeelectrically insulated.

As a result of this unsatisfactory contact with the current collector orwith the membrane, more platinum than would actually be necessary forachieving a particular electric output is needed. In practice, theamount of platinum used is from about 0.5 to 4 mg/cm² of membrane area.This corresponds to a number of 100 g platinum for a practical vehiclehaving a motor power of 40-50 kW.

A further, significant reason for the increased platinum requirement isthe production process predominantly employed in the past for theplatinum/carbon mixture. In this process, the solution of a reducible orprecipitatable platinum compound is applied to the carbon support byimpregnation or spraying. Subsequently, the platinum compound isconverted into finely divided platinum or platinum oxide particles byprecipitation and/or chemical reduction, frequently resulting information of relatively large particles having a diameter of up to a few10 s to 100 nanometers. This causes a reduction in the catalyticactivity as a result of the decrease in the specific surface area of theplatinum. This can be illustrated by the following example: a cluster,which in the interests of simplicity can be thought of as a queue, madeof up atoms of a metal having a given diameter of 0.25 nanometerscontains approximately 87% surface atoms at an edge length of 1nanometer, 49% surface atoms at an edge length of 2.5 nanometers and0.14% surface atoms at an edge length of 10 nanometers.

It is also known that a platinum catalyst on a carbon support losessurface area under customary operation conditions, i.e. at elevatedtemperature. This loss is due to the fact that the platinum particlesmigrate on the support surface and can combine with other particles,i.e. recrystallize to form larger particles. This effect is morepronounced, the smaller the platinum particles. From this point of viewit is desirable to reduce the migration velocity of the platinumparticles by embedding them in a polar microenvironment which interactsstrongly with the carbon support.

In summary, it may be said that in order to obtain a functional membranefuel cell it is necessary, firstly, to achieve a high dispersion of thecatalytically active metal centers, secondly to ensure unhinderedtransport of starting materials, products and also protons and electronsand thirdly to reduce the recrystallization of the metal particles toform larger particles on the carbon support.

It is an object of the present invention to provide water-soluble,stabilized metal colloids comprising ultrafine particles of platinum. orplatinum metals and also a process for preparing them. These colloidsshould be suitable as catalysts, in particular for fuel cells. Whenusing such metal colloids as catalysts for membrane-electrode units(MEAs), the platinum particles should be in good proton-conductingcontact with the membrane and exhibit a reduced tendency torecrystallize.

The present invention achieves this object and thus provideswater-soluble metal colloids comprising one or more platinum compoundsand, if desired, one or more compounds of Rh, Ru, Ir or Pd, where themetal colloids are stabilized by a proton-conducting protective colloid.According to the invention, the protective colloids used arewater-soluble or solubilizable cation-exchange polymers.

Furthermore, the present invention provides a process for preparingthese metal colloid solutions by reacting a platinum compound and, ifdesired, one or more compounds of Rh, Ru, Ir or Pd with a reducingagent. To stabilize the metal colloid solutions, use is made of at leastone cation-exchange polymer, and the reduction is either carried out inthe presence of the cation-exchange polymer or the cation-exchangerpolymer is added to the solution after the reduction step. Thestabilized metal colloid (sol) can subsequently be purified byreprecipitation and/or be concentrated by evaporation.

When the metal colloids of the invention are used as catalysts, theultrafine particles comprising a proton-conductive polymer and envelopedin a microstructure are uniformly distributed and immobilized on thesurface or in the surface region of a carbon support so that theplatinum particles can, in the subsequent construction of the MEA, bebrought into improved proton-conducting contact with the membrane andexhibit a reduced tendency to recrystallize.

To prepare the metal colloid solutions of the invention, the metalcompounds of platinum, rhodium, ruthenium, palladium and iridium to beused are employed as starting materials in the form of solublecompounds, in particular water-soluble salts. Examples arehexachloroplatinic(IV) acid hydrate, hydroxydisulfitoplatinic acid,platinum nitrate, hexachloroiridic(IV) acid hydrate, palladium(II)acetate, iridium(III) acetylacetonate, ruthenium(III) acetylacetonate,ruthenium(III) nitrate, rhodium(III) chloride hydrate, to name only afew. The metal compounds are used in concentrations of from about 0.1 to100 g per liter, preferably from 1 to 50 g per liter, based on thesolvent.

In a preferred embodiment, the ratio of Pt to other platinum metals is99-60% by weight of platinum to 1-40% by weight of Rh, Ru, Ir and/or Pd.

The cation-exchange polymers used for preparing the metal colloidsolutions possess strongly acidic, easily dissociable groups, forexample carboxylic acid groups, sulfonic acid groups or phosphonic acidgroups. A characteristic of the polymers used is thus the ability ofcations and particularly protons to move readily in the polymer matrix,particularly in the swollen state.

The cation-exchange polymers which can be used according to theinvention may be selected from various classes of chemical substances,for example sulfonated polyaryl ether ketones, sulfonated polyethersulfones, sulfonated polyphenylene sulfides, sulfonatedacrylonitrile-butadiene-styrene copolymers (ABS), poly(styrenesulfonic)acids, poly(α,β,γ-trifluorostyrenesulfonic acids) andpoly[perfluoroethylene-co-2-(1-methyl-2-vinyloxyethoxy)ethanesulfonicacid] and perfluorinated cation-exchange resins and otherhigh-performance polymers having a similar structure.

The polymers belonging to the group of sulfonated polyaryl ether ketonesare made up of phenylene radicals which are linked via ether or ketonegroups and preferably bear sulpho groups in the ether subunits. Examplesof such polymers are the sulfonated polyether ketones (PEK) (1),sulfonated polyether ether ketones (PEEK) (2), sulfonated polyetherketoneketones (PEKK) (3), sulfonated polyether ether ketone ketones(PEEKK) (4)⁻ and sulfonated polyether ketone ether ketone ketones(PEKEKK) (5) shown in the following formulae:

The unsulfonated parent polymers are known, for example, under the tradenames Hostatec®, Victrex® or Ultrapek®.

The polymers belonging to the group of sulfonated polyaryl ethersulfones are made up of phenylene radicals which are linked via ether orsulfone groups and bear sulfonic acid groups in the ether subunits, forexample the sulfonated polyether sulfone (PES) (6):

Such polymers are obtained using unsulfonated base polymers as areknown, for example, under the tradenames polyether sulfone Victrex 200P®, polyether sulfone Victrex 720 P®. Polyaryl sulfone Radel®, polyethersulfone Astrel®, polysulfone or Udel® as starting materials.

The sulfonic acid groups can be introduced by methods known per se byreacting the base polymers with sulfuric acid, oleum or chlorosulfonicacid as described in JP-A-043 107 732.

The degree of sulfonation indicates the percentage of monomer unitswhich bear a sulfonic acid group. Polyether ketones or polyethersulfones which are suitable for the process of the invention preferablyhave a degree of sulfonation in the range from 20 to 95%, in particularin the range from 40 to 85%.

Perfluorinated cation-exchange resins which can be used according to theinvention are, for example, copolymers of tetrafluoroethylene andperfluorinated vinyl ethers having a terminal sulfonic acid group,phosphonic acid group or carboxylic acid group. Formula (7) shows atypical structure of perfluorinated cation-exchange resins, withoutimplying that the fluoro-polymers which can be used according to theinvention are restricted to this formula:

Examples of commercially available perfluorinated cation-exchange resinsarepoly[perfluoroethylene-co-2-(1-methyl-2-vinyloxyethoxy)ethanesulfonicacid] (x=1, y=2) and compounds having similar structures. The productsare obtainable under the tradenames Aciplex-S® (Asahi Chemical) orNafion® (E.I. DuPont de Nemours) or as experimental membrane (DowChemical).

To prepare the metal colloid solutions, soluble or solubilizablecation-exchange polymers can be used as protective colloids. Thesolubility of the polymers in water or lower aliphatic alcohols, forexample methanol or ethanol, can be controlled via the degree ofpolymerization and via the number of acid groups. Aqueous alcoholicsolutions or sols of Nafion are commercially available. If desired,colloidal solutions can also be prepared from commercial, perfluorinatedcation-exchange membranes by heating for a number of hours inN-methylpyrrolidone; these can subsequently be diluted with water.

Solutions of sulfonated polyether ketones can be prepared by dissolvingthem in, for example, N-methylpyrrolidone, dimethyl sulfoxide,dimethyl-formamide or dimethylacetamide, and these solutions cansubsequently be diluted with water or lower aliphatic alcohols.

Cation-exchange polymers which are suitable according to the inventionhave an ion-exchange capacity in the range from 0.5 to 5 meq/g, inparticular in the range from 0.8 to 3.5 meq/g. Preference is generallygiven to polymers which have relatively high ion-exchange capacities andtend to dissolve readily in water and/or polar solvents and have a highswelling capability.

The cation-exchange polymers are used in amounts of from 5 to 400% byweight, preferably from 10 to 2,000% by weight, based on the metalpresent (Pt, Ir, Rh, Ru and Pd).

The reduction can be carried out in water or in a mixture of water andone or more water-miscible organic solvents or in the absence of waterin an organic solvent.

Examples of suitable solvents are methanol, ethanol, ethylene glycol,tetrahydrofuran, dimethoxyethane, acetone, N-methylpyrrolidone,dimethyl-formamide and dimethylacetamide. The metal colloid solutionsare preferably prepared in water (hydrosols) or in water with additionof from 1 to 50% by weight, preferably from 5 to 25% by weight, of anorganic solvent.

Suitable reducing agents are all customary reducing agents which have asufficently negative reduction potential, for example hydrogen, sodiumborohydride, monohydric or dihydric alcohols such as ethanol or ethyleneglycol, or hydroxymethanesulfinic acid sodium salt. The preferredreducing agent is hydroxymethanesulfinic acid sodium salt (Rongalit®).

The reducing agent is generally used in a stoichiometric amount based onthe metal compound(s), but preferably in excess. The excess can be, forexample, from 10 to 100 mol %.

The sols are preferably prepared at temperatures in the range from 0 to200° C., in particular from 20 to 100° C. The components can generallybe added in any order, and it is useful to stir the mixture so as to aidmixing. In the preferred procedure, the reducing agent. is added last.If-the cation-exchange polymer is added only after the reduction, it hasto be added before agglomeration commences.

The soluble metal colloids of the invention are soluble in water or anorganic solvent, with “soluble” also being used in the sense of“solubilizable” i.e. forming sols. The solubility is at least 50 g/l andis usually in the range from 50 to 200 g/l, in particular in the rangefrom 70 to 150 g/l.

The metal colloids stabilized by cation-exchange polymers are novelcompounds of relatively uniform composition. On the basis of studies bytransmission electron microscopy (TEM), the particles obtained have avery narrow size distribution. Typically 90% of the particles deviate byless than 20% from the mean diameter. The diameter of the metal coredepends to some extent on the type and amount of the stabilizer used. Itis generally less than 3 nanometers, mostly less than 2 nanometers. Inmany cases, the diameter of the metal core is about 1 nanometer or less.

The total diameter R_(h) of the particles including the protectivecolloid shell has been found to be in the range from about 2 to 4nanometers by means of dynamic light scattering.

The platinum-cation-exchange polymer complexes after reprecipitation andisolation as solid contain about 55-65% by weight of metal(s) (Pt, Pd,Ir, Rh, Ru).

The metal colloids of the invention are suitable as catalysts, inparticular for fuel cells. For this purpose, for example, a finelydivided support, e.g. of carbon, carbon black or graphite, is broughtinto contact with the metal colloid solution of the invention and thecatalyst is separated from the liquid phase in a manner known per se byfiltration or centrifugation.

To produce the platinum/carbon black mixture, metal concentrations of atleast 10 g/liter are generally desirable. The metal colloid solutions(sols) obtained according to the invention can, if desired, beconcentrated by gently distilling off water and/or the solvent. Ifnecessary, the sols obtained according to the invention can be purifiedby reprecipitation in a manner known per se and can, if desired, beconcentrated at the same time. Precipitation of a colloidally dissolvedplatinum-cation-exchange polymer complex can be carried out by additionof acetone or isopropanol. The platinum-cation-exchange polymer gelsobtained can be redissolved in water, giving metal concentrations of atleast 50 g/liter.

To produce catalysts, the aqueous metal colloid solutions prepared asdescribed above are brought into contact with a fine powder ofconductive support material and the liquid phase is subsequentlyseparated off. This results in immobilization of the platinum particlessurrounded by an inherent, proton-conducting envelope on the supportparticle. It has been found that the platinum-cation-exchange polymercomplexes of the invention are preferentially deposited on the surfaceor in surface-near regions of the support and have good adhesion to thesupport.

This makes if possible to improve the transport of protons between thecatalytic centers in the electrode layer of the anode or cathode,respectively, and the membrane.

The support comprises, in particular, finely divided carbon, carbonblack or graphite. Preference is given to using. specific, electricallyconductive carbons (carbon black) which are commercially available, forexample ®Vulcan XC 72R.

The carbon supports used can, before or after being loaded with thenanosize platinum particles of the invention, be additionally treatedwith materials such as proton-conducting polymers (U.S. Pat. No.4,876,115).

The carbon support can be loaded, for example, by introducing the metalcolloid solution with mixing into a suspension of the support in wateror a water/alcohol mixture, stirring the suspension for a further periodand isolating the platinum/carbon mixture by filtration orcentrifugation.

The metal colloid solutions produced according to the invention are verystable and contain particles having a diameter of typically 1 nanometeror less. This achieves an extraordinarily high dispersion of theexpensive noble metals.

The microenvironment of a rigid, polar cation-exchange polymeradditionally effects good stabilization of the catalytic centers on thesupport and inhibits recrystallization of the particles on the support.

The cation-exchange polymers, in particular the perfluorinatedcation-exchange resins, utilized for enveloping the catalytically activecenters have a good solvent capacity for the gaseous fuels or oxygen. Asa result, transport of the reactants to the centers is not hindered.

The cation-exchange polymers generally have a high swelling capabilitywhich can be adjusted via the molecular weight and the ion-exchangecapacity. When using the metal colloid solutions of the invention ascatalysts, it is thus possible to improve the water balance of theelectrodes, which differs between anode and cathode, by adapting theswelling capability of the cation-exchange polymers used.

EXAMPLE 1

570 ml of deionized water, 30 g of a 5% strength solution of sulfonatedHostatec® (polyether ketone (PEEKK), molecular weight M_(n): about40,000, manufacturer: Hoechst AG, Frankfurt am Main; degree ofsulfonation: 65%) in N-methylpyrrolidone and 2.50 g (about 5 mmol) ofhexachloroplatinic(IV) acid hydrate (platinum content: about 40%) wereplaced in a 2 1 conical flask. 5% strength ammonia solution was addeddropwise until a pH of 7.0 had been reached. While stirring vigorouslyat 90-95° C., a solution of 2.50 g (21 mmol) of hydroxymethanesulfinicacid sodium salt (Rongalit) in 20 ml of deionized water was then added.The solution briefly became lighter in color and then became darkreddish brown. It was allowed to stand for 15 hours at room temperature,after which the hydrosol obtained was filtered through a G4 glass frit,the filtrate was admixed with 750 ml of acetone, stirred for 5 minutesand the precipitate which formed was allowed to settle for 3 hours.After decanting off most of the supernatant liquid, the remainder wascentrifuged for 15 minutes at 7,000 rpm. The supernatant liquids fromthe decantation and centrifugation were admixed with 500 ml of acetoneand centrifuged. The two centrifugation residues were combined anddissolved in 30 ml of deionized water, precipitated by addition of 90 mlof acetone and centrifuged again. The moist residue obtained wasdis-solved in water to give 20 g.

TEM analysis of the particles (transmission electron microscope: PhilipsCM 30; a sample of the sol was applied to a carbon-coated copper gauze)gave a particle size of 1 nanometer. 5.0 g of the hydrosol obtained wereevaporated and dried over concentrated sulfuric acid in a vacuumdesiccator. Analysis of the solid obtained gave 63% of platinum(ICP-OES) and 10.6% of sulfur (combustion analysis/IR detection). Thedried gel redissolved in water.

EXAMPLE 2

500 ml of deionized water, 50 g of a 2% strength solution of sulfonatedHostatec® (polyether ether ketone ketone (PEEKK), molecular weightM_(n): 40,000, manufacturer: Hoechst AG, Frankfurt am Main; degree ofsulfonation: 74.2%) in N-methylpyrrolidone and 2.50 g (about 5 mmol) ofhexachloroplatinic(IV) acid hydrate (platinum content: about 40%) wereplaced in a 2 I conical flask. 5% strength ammonia solution was addeddropwise until a pH of 7.0 had been reached. While stirring vigorouslyat 90-95° C., a solution of 2.50 g (21 mmol) of hydroxymethanesulfinicacid sodium salt (Rongalit) in 20 ml of deionized water was then added.The solution briefly became lighter in color and then became darkreddish brown. It was allowed to cool to room temperature and left tostand for 20 hours, and the sol was then admixed with 600 ml of acetone,stirred for 5 minutes and the precipitate which formed was allowed tosettle for 4 hours. After decanting off most of the supernatant liquid,the remainder was centrifuged for 15 minutes at 7,000 rpm. Thecentrifugation residue was dissolved in 50 ml of deionized water,precipitated by addition of 100 ml of acetone and centrifuged again. Themoist residue obtained was dissolved in 40 ml of water and, afteraddition of 10 g of N-methylpyrrolidone, concentrated to 20 g.

TEM analysis of the particles (transmission electron microscope: PhilipsCM 30; a sample of the sol was applied to a carbon-coated copper gauze)gave a particle size of less than 1 nanometer. A sample of the solobtained was precipitated by means of acetone, centrifuged and driedover concentrated sulfuric acid in a vacuum desiccator. Analysis of thesolid obtained gave 63% of platinum (ICP-OES) and 11.5% of sulfur(combustion analysis/IR detection). The dried gel redissolved in water.

EXAMPLE 3

500 ml of deionized water and 170 g of N-methylpyrrolidone were placedin a 2 I conical flask. 50 g of a 2% strength solution of sulfonatedVictrex® (polyether ether ketone (PEEK), molecular weight M_(n): about80,000, manufacturer: ICI; degree of sulfonation: 50.7%) inN-methylpyrrolidone and 2.50 g (about 5 mmol) of hexachloroplatinic(IV)acid hydrate (platinum content: about 40%) were added, followed bydropwise addition of 5% strength ammonia solution until a pH of 7.0 hadbeen reached. While stirring vigorously at 90-95° C., a solution of 2.50g (21 mmol) of hydroxymethanesulfinic acid sodium salt (Rongalit ) in 20ml of deionized water was then added. The solution briefly becamelighter in color and then became dark reddish brown. It was allowed tocool to room temperature and left to stand for 20 hours, and the sol wasthen admixed with 600 ml of acetone, stirred for 5 minutes and theprecipitate which formed was allowed to settle for 4 hours. Afterdecanting off most of the supernatant liquid, the remainder wascentrifuged for 15 minutes at 7000 rpm. The centrifugation residue wasdissolved in 50 ml of deionized water, precipitated by addition of 100ml of acetone and centrifuged again. The moist residue obtained wasdissolved in 40 ml of water and, after addition of 10 g ofN-methylpyrrolidone, concentrated to 20 g.

TEM analysis of the particles (transmission electron microscope: PhilipsCM 30; a sample of the sol was applied to a carbon-coated copper gauze)gave a particle size of less than 1 nanometer. A sample of the solobtained was precipitated by means of acetone, centrifuged and driedthoroughly over concentrated sulfuric acid in a vacuum desiccator.Analysis of the solid obtained gave 63% of platinum (ICP-OES) and 10.9%of sulfur (combustion analysis/IR detection). The dried gel redissolvedin water.

EXAMPLE 4

500 ml of deionized water were placed in a 2 I conical flask. 50 g of a2% strength solution of Nafion® 117 in N-methylpyrrolidone (Nafion® 117is a commercially available perfluorinated cation-exchange membrane fromE.I. DuPont de Nemours. 1.94 g of Nafion® 117 pieces were heated at190-195° C. in 100 g of N-methylpyrrolidone for 16 hours while stirringand the resulting solution was evaporated to dryness under reducedpressure. The residue (1.0 g) was dissolved in 50 g ofN-methylpyrrolidone) and 1.00 g (about 2 mmol) of hexachloroplatinic(IV)acid hydrate (platinum content: about 40%) were added, followed bydropwise addition of 5% strength ammonia solution until a pH of 7 hadbeen reached. While stirring vigorously at 90-95° C., a solution of 1.00g (8.5 mmol) of hydroxymethanesulfinic acid sodium salt (Rongalit ) in20 ml of deionized water was then added. The solution briefly becamelighter in color and then became dark reddish brown. It was allowed tocool to room temperature and left to stand for 20 hours, and thehydrosol was then admixed with 600 ml of acetone, stirred for 5 minutesand the precipitate which formed was allowed to settle for 3 hours.After decanting off most of the supernatant liquid, the remainder wascentrifuged for 15 minutes at 7000 rpm. The centrifugation residue wasdried over concentrated sulfuric acid in a vacuum desiccator.

Analysis of the solid obtained gave 66% of platinum (ICP-OES) and 10.2%of sulfur (combustion analysis/IR detection). The dried gel remainedwater-soluble for some days.

EXAMPLE 5

500 ml of deionized water were placed in a 2 l conical flask. 1.00 g(about 2 mmol) of hexachloroplatinic(IV) acid hydrate (platinum content:about 40%) was added, followed by dropwise addition of 5% strengthammonia solution until a pH of 7 had been reached. While stirringvigorously at 90-95° C., 2.0 g of a 5% strength solution of Nafionperfluorinated ion-exchange powder in a mixture of water and loweraliphatic alcohols (procured from Aldrich-Chemie GmbH & Co KG, D-89552Steinheim) and subsequently a solution of 1.00 g (8.5 mmol) ofhydroxymethanesulfinic acid sodium salt (Rongalit®) in 20 ml ofdeionized water were added. The solution briefly became lighter in colorand then became dark reddish brown. It was allowed to cool to roomtemperature and left to stand for 6 hours, and the hydrosol was thenadmixed with 1000 ml of acetone, stirred for 5 minutes and theprecipitate formed was allowed to settle for 15 hours. After decantingoff most of the supernatant liquid, the remainder was centrifuged for 15minutes at 7000 rpm. The centrifugation residue was all dissolved inwater to give 10.0 g of reddish brown sol. TEM analysis of the particles(transmission electron microscope: Philips CM 30; a sample of the solwas applied to a carbon-coated copper gauze) gave a particle size ofless than 1 nanometer.

EXAMPLE 6

500 ml of deionized water were placed in a 1l conical flask. 2.00 g(about 4 mmol) of hexachloroplatinic(IV) acid hydrate (platinum content:about 40%), 0.40 g (about 0.8 mmol) of hexachloroiridic(IV) acidhydrate) and 10 g of a 5% strength solution of sulfonated Victrex®(polyether ether ketone (PEEK), molecular weight M_(n): about 80,000,manufacturer: ICI; degree of sulfonation: 50.7%) in N-methylpyrrolidonewere added. The solution was admixed with 5% strength ammonia solutionuntil a pH of 7 had been reached. While stirring vigorously at 95-98°C., a solution of 1.77 g (about 15 mmol) of hydroxymethanesulfinic acidsodium salt (Rongalit®) in 20 ml of deionized water was added. Thesolution briefly became lighter in color and then became dark brown. Itwas allowed to cool to room temperature and left to stand for 20 hours,and the hydrosol formed was admixed with 3500 ml of acetone in a 4 lglass beaker, stirred for 5 minutes and the precipitate formed wasallowed to settle for 20 hours. After decanting off most of thesupernatant liquid, the remainder was centrifuged for 15 minutes at 7000rpm. The centrifugation residue was dissolved in 100 ml of water andreprecipitated by addition of 150 ml of acetone and centrifuged. Theresidue was dissolved in water to give 20.0 g.

EXAMPLE 7

500 ml of deionized water were placed in a 1 l conical flask. 2.00 g(about 4 mmol) of hexachloroplatinic(IV) acid hydrate (platinum content:about 40%), 0.30 g (about 1 mmol) of ruthenium(III) chloride hydrate and10 g of a 5% strength solution of sulfonated Victrex® (polyether etherketone (PEEK), molecular weight M_(n): about 80,000, manufacturer: ICI;degree of sulfonation: 50.7%) in N-methylpyrrolidone were added. Thesolution was admixed with 5% strength ammonia solution until a pH of 7had been reached. While stirring vigorously at 95-98° C., a solution of1.77 g (about 15 mmol) of hydroxymethanesulfinic acid sodium salt(Rongalit®) in 20 ml of deionized water was added. The solution brieflybecame lighter in color and then became dark brown. It was allowed tocool to room temperature and left to stand for 20 hours, and thehydrosol formed was admixed with 3000 ml of acetone in a 4 l glassbeaker, stirred for 5 minutes and the precipitate formed was allowed tosettle for 5 hours. After decanting off most of the supernatant liquid,the remainder was centrifuged for 15 minutes at 7000 rpm. Thecentrifugation residue was dissolved in 100 ml of water andreprecipitated by addition of 150 ml of acetone and centrifuged. Theresidue was dissolved in water to give 20.0 g.

EXAMPLE 8

2.00 g of Vulcan XC 72R (Manufacturer: Cabot B. V., Rozenburg, TheNetherlands), 25 ml of water and 5 ml of methanol were placed in a 100ml round-bottomed flask which contained 5 porcelain spheres (diameter:10 mm), and were mixed by rotation on a rotary evaporator at 100 rpm for4 hours. While continuing the rotation, 5.0 g of platinum sol preparedas described in Example 1 using a sulfonated PEEKK protective colloidand diluted with 5 ml of water was pumped at 20-25° C. over a period of0.5 hours into the uniform suspension obtained. The suspension wassubsequently rotated for another 3 hours. The coated carbon was filteredoff with suction (blue band filter, Schleicher & Schüll) and dried overconcentrated sulfuric acid in a vacuum desiccator. The weight obtainedwas 2.24 g. Analysis of the catalyst obtained gave 66% of platinum(ICP-OES). TEM analysis (transmission electron microscope: Philips CM30; the particles were applied to a carbon-coated copper gauze) of thecatalyst particles indicated a uniform distribution of the platinumparticles which had a diameter of about 1 nanometer.

EXAMPLE 9

The procedure was as in Example 8. 2.00 g of Vulcan XC 72R weresuspended in 20 ml of water and 5 ml of methanol. While continuing therotation, 6.7 g of Nafion-stabilized platinum sol prepared as describedin Example 5 using a perfluorinated cation-exchange resin as protectivecolloid were pumped into this suspension at 20-25° C. over a period of 1hour. The suspension was rotated for another two hours and subsequentlycentrifuged. The centrifugation residue was dried over concentratedsulfuric acid in a vacuum desiccator. The weight obtained was 2.20 g.Analysis of the catalyst obtained gave 8.1% of platinum (ICP-OES). TEManalysis (transmission electron microscope: Philips CM 30; the particleswere applied to a carbon-coated copper gauze) of the catalyst particlesindicated a fine coating of platinum particles which had a diameter ofabout 1-2 nanometer.

EXAMPLE 10

The procedure was as in Example 8. 2.00 g of Vulcan XC 72R weresuspended in 20 ml of water and 5 ml of methanol. While continuing torotate, 6.7 g of platinum sol prepared as described in Example 3 using asulfonated PEEK protective colloid was pumped in at 20-25° C. over aperiod of 1 hour. The suspension was rotated for another 2 hours andsubsequently centrifuged. The centrifugation residue was dried overconcentrated sulfuric acid in a vacuum desiccator. The weight obtainedwas 2.30 g. Analysis of the catalyst obtained gave 13% of platinum(ICP-OES).

TEM analysis (transmission electron microscope: Philips CM 30; theparticles were applied to a carbon-coated copper gauze) of the catalystparticles indicated a coating of very fine platinum particles whosediameter was not more than 1 nanometer.

EXAMPLE 11

An immobilization was carried out using a method analogous to Example 8.The support material used was 2.00 g of Vulcan XC 72R (manufacturer:Cabot B.V., Rozenburg, The Netherlands) which had been treatedbeforehand with a solution of sulfonated Hostatec®. 6.6 g (about 0.33 gof platinum, stabilizer: sulfonated Hostatec) of sol concentrateprepared as described in Example 1 were used. The weight obtained was1.97 g.

EXAMPLE 12

An immobilization was carried out using a method analogous to Example 8.The support material used was 2.00 g of Vulcan XC 72R (manufacturer:Cabot B.V., Rozenburg, The Netherlands) which had been treatedbeforehand with a solution of polybenzimidazole. 6.6 g (about 0.33 g ofplatinum, stabilizer: sulfonated Victrex®) of sol concentrate preparedas described in Example 3 were used. The weight obtained was 2.06 g.

EXAMPLE 13

An immobilization was carried out using a method analogous to Example 8.The support material used was 2.00 g of Vulcan XC 72R (manufacturer:Cabot B. V., Rozenburg, The Netherlands) which had been treatedbeforehand with a solution of polybenzimidazole. 6.6 g (about 0.33 g ofplatinum, stabilizer: Nafion®) of sol concentrate prepared as describedin Example 5 were used. The weight obtained was 1.68 g.

EXAMPLE 14

An immobilization was carried out using a method analogous to Example 8.The support material used was 2.00 g of Vulcan XC 72R (manufacturer:Cabot B. V., Rozenburg, The Netherlands) which had been treatedbeforehand with a solution of sulfonated Victrex®. 6.6 g (about 0.33 gof platinum, stabilizer: sulfonated Victrex®) of sol concentrateprepared as described in Example 3 were used. The weight obtained was2.06 g. Analysis of the catalyst obtained gave 3.8% of platinum(ICP-OES).

What is claimed is:
 1. A process for preparing metal colloid solutionsby reacting a platinum compound and, if desired, one or more compoundsof Rh, Ru, Ir or Pd with a reducing agent, wherein at least onecation-exchange polymer selected from the group consisting of:sulfonated polyaryl ether ketones, sulfonated polyether sulfones,sulfonated polyphenylene sulfides and sulfonatedacrylonitrile-butadiene-styrene copolymers (ABS)poly(α,β,γ-trifluorostyrene-sulfonic acids) andpoly(polyfluoroethylene-co-2-(1-methyl-2-vinyloxyethoxy)ethane sulfonicacid) is used for stabilizing the metal colloid solution.
 2. The processas claimed in claim 1, wherein the cation-exchange polymer is added tothe solution after the reduction step.
 3. The process as claimed inclaim 1, wherein the reduction is carried out in water, a mixture ofwater and at least one water-miscible organic solvent or in the absenceof water in an organic solvent.
 4. The process as claimed in claim 1,wherein the platinum compounds used are water-soluble compounds.
 5. Theprocess as claimed in claim 1, wherein the concentration of the Ptcompound or the Pt and Rh, Ir, Ru and/or Pd compounds based on thesolvent is in the range from 0.1 to 100 g/l of solvent.
 6. The processas claimed in at claim 1, wherein the reducing agent used is sodiumborohydride, hydrogen, hydroxymethanesulfinic acid sodium salt or amonohydric or dihydric alcohol.
 7. The process as claimed in claim 1,wherein the preparation of the metal colloids is carried out attemperatures in the range from 0 to 200° C.
 8. The process as claimed inclaim 1, wherein the metal colloid solution stabilized with thecation-exchange polymer is purified or concentrated in a subsequentstep.
 9. The process as claimed in claim 1, wherein the cation-exchangepolymer used has an ion-exchange capacity in, the range from 0.5 to 5meq/g.
 10. The process as claimed in claim 9, wherein the degree ofsulfonation of the sulfonated cation-exchange polymers is in the rangefrom 20 to 95%.
 11. The process as claimed in claim 10, wherein thereducing agent used is hydroxymethanesulfinic acid sodium salt.
 12. Awater-soluble metal colloid comprising one or more platinum compoundsand, if desired, one or more compounds of Rh, Ru, Ir or Pd, wherein themetal colloid is stabilized by a cation-exchange polymer, selected fromthe group consisting of: sulfonated polyaryl ether ketones, sulfonatedpolyether sulfones, sulfonated polyphenylene sulfides, sulfonatedacrylonitrile-butadiene-styrene copolymers (ABS),poly(α,β,γ-trifluorostyrene-sulfonic acids), andpoly(perfluoroethylene-co-2-(-1-methyl-2-vinyloxyethoxy)ethanesulfonicacid).
 13. A water-soluble metal colloid as claimed in claim 12, whereinthe cation-exchange polymer used has an ion-exchange capacity in therange from 0.5 to 5 meq/g.
 14. A water-soluble metal colloid as claimedin claim 12, wherein the metal colloid has a solubility in water of atleast 50 g/l, based on the metal concentration.
 15. A water-solublemetal colloid as claimed in claim 12, wherein the metal colloid is on asupport material.
 16. A water-soluble metal colloid as claimed in claim15, wherein the support material comprises carbon.